3D depth merging

Hey guys, I am rendering a lot of arial animation of buildings etc. and getting everything to render in 3D is a RAM miracle. I know I can cut corners rendering in the 3D app but is there another way to split up the file?

I was wondering of rendering the buildings with a 16 bit depth channel. The trees in a separate pass with another 16 bit depth channel.
I wonder if there is a way to use channel arithmatic (or a transfer mode perhaps) to merge the two depth file so that AE would know if a tree would block the buildings in different spaces, the closest Zdepth pixel wins.

Am I blabbing or is this possible?_B